The present invention relates to power converters. More particularly, the present invention relates to a DC-DC converter capable of having zero-voltage-switching over the full load range of operation.
A phase-modulated full-bridge converter (PMC) is a common topology used for DC-DC conversion. The PMC circuit typically includes an inductor and capacitor connected in series across the input terminals of the circuit. Four switching elements are connected together to form a left-leg branch and a right-leg branch, wherein each branch comprises two switching elements connected in series across the capacitor. A primary winding of a transformer connects the left-leg branch with the right-leg branch at the series connection of the switching elements of each of the branches. A secondary winding of the transformer is provided to an output circuit comprising a full bridge rectifier and a low-pass filtering circuit.
The main advantage of the PMC circuit is zero-voltage-switching of the switching elements while still operating at a constant switching frequency, which allows a simple control circuit. However, the main disadvantage of the conventional PMC circuit is that the zero-voltage-switching characteristics are load dependent, and achieving zero-voltage-switching below, for example, one-half load causes unacceptable high conduction losses.
From the view point of zero-voltage-switching, the left-leg branch switching elements and the right-leg branch switching elements of the PMC circuit operate under significantly different conditions. During transition of the right-leg branch, the transformer primary current does not change direction and remains in the proper direction to discharge the appropriate switch capacitances in order to achieve zero-voltage turn-on. However, during transition of the left-leg branch, the transformer primary current reduces and eventually changes polarity. Hence, the energy available for charging/discharging the appropriate switch capacitance is less. Therefore, achieving zero-voltage-switching for the left-leg branch switching elements is more difficult. Also, the energy available is a function of the load current and at light loads zero-voltage-switching is lost.
Two techniques have been advanced to increase the zero-voltage-switching load range. A first technique includes adding an external inductor in series with the primary of the transformer. The second technique includes increasing the magnetizing current of the transformer. Nevertheless, both of these techniques result in high conduction losses. Therefore, zero-voltage-switching in the conventional PMC circuit at light loads is obtained only at the expense of increased conduction losses and increased VA ratings of the magnetic components. Accordingly, as a compromise between the switching and conduction losses, the PMC circuit is usually designed to achieve zero-voltage-switching only above a certain load, typically about sixty percent of the full-load. Below this limit, the converter typically operates in a hard-switch mode.
There thus is a continuing need to improve the zero-voltage-switching range of a DC-DC converter.